1. Technical Field of the Invention
The present invention relates generally to spark plugs for use in internal combustion engines of automobiles and cogeneration systems.
More particularly, the invention relates to a spark plug for an internal combustion engine which can induce spark discharges with a low discharge voltage and has an excellent self-cleaning capability.
2. Description of the Related Art
FIG. 19 shows a conventional spark plug 9 for an internal combustion engine of an automobile or a cogeneration system. As shown in the figure, the spark plug 9 includes an insulator 92, a center electrode 93, a metal shell 94, and a ground electrode 95. The center electrode 93 is secured in the insulator 92. The insulator 92 is retained in the metal shell 94 such that an end 1 thereof protrudes from the metal shell 94. The ground electrode 95 is fixed to the metal shell 94 and faces the center electrode 93 through a spark gap 91 in the longitudinal direction Z-Z of the spark plug 9.
In normal conditions of the spark plug 9, sparks can be discharged across the spark gap 91 by applying a discharge voltage (i.e., the voltage required to induce spark discharges) between the center and ground electrodes 93 and 95.
However, when the surface of the insulator 92 is fouled with carbon that is electrically conductive, the insulation resistance between the center and ground electrodes 93 and 95 will be decreased, thus making it difficult for the spark plug 9 to induce spark discharges across the spark gap 91.
Accordingly, it is desirable for the spark plug 9 to have a self-cleaning capability to clean the surface of the insulator 92 by burning off the carbon that has deposited on the surface.
To impart such a self-cleaning capability to the spark plug 9, according to an approach disclosed in Japanese Patent First Publication No. 2004-6250, the center electrode 93 is configured to include a base member 931 and a thin member 932. The base member 931 is fitted in a central bore 923 of the insulator 92 and has an end 933 that is positioned inside the central bore 923 in proximity to the end 921 of the insulator 92. The thin member 932 is thinner than the base member 931 and connected to the end 933 of the base member 931.
With such a configuration, when the surface of the insulator 92 is clean, spark discharges A can be induced between the thin member 932 of the center electrode 93 and a protruding member 952 of the ground electrode 95. On the other hand, when the surface of the insulator 92 is fouled with carbon, spark discharges B can be induced between the base member 931 of the center electrode 93 and the protruding member 952 of the ground electrode 95. The spark discharges B pass the inner surface 924 of the insulator 92 defining the central bore 923, thereby burning off the carbon that has deposited on the inner surface 924. Consequently, the spark plug 9 can be self-cleaned.
However, with the above configuration, there is a contradiction between enhancement of the self-cleaning capability and reduction of the discharge voltage of the spark plug 9.
More specifically, in recent years, direct injection engines and high compression ratio engines have been developed to meet the requirement of improving fuel economy.
In direct injection engines, it is difficult for fuel to vaporize due to the direct injection thereof into combustion chambers. As a consequence, when the spark plug 9 is used in a direct injection engine, it is easy for carbon to deposit on the surface of the insulator 92. Accordingly, the spark plug 9 is required to have a high self-cleaning capability.
To meet the above requirement, referring to FIG. 20, it is necessary for an end edge 953 of the protruding member 952 of the ground electrode 95 to be arranged as close to the inner surface 924 of the insulator 92 as possible. However, this arrangement causes, at the same time, the end edge 953 to get away from an end edge 934 of the thin member 932 of the center electrode 93, thereby increasing the discharge voltage of the spark plug 9.
On the other hand, when the spark plug 9 is used in a high compression ratio engine, it is easy for the discharge voltage of the spark plug 9 to be increased due to the high compression pressure. The increase in the discharge voltage of the spark plug 9 may cause dielectric breakdown of the insulator 92. Accordingly, it is required to reduce the discharge voltage of the spark plug 9.
To meet the above reduction requirement, referring to FIG. 21, it is necessary for the end edge 953 of the protruding member 952 of the ground electrode 95 to be arranged as close to the end edge 934 of the thin member 932 of the center electrode 93 as possible. However, this arrangement may cause, at the same time, the end edge 953 to get away from the inner surface 924 of the insulator 92, thereby lowering the self-cleaning capability of the spark plug 9.